1. Field of the Invention
This invention relates generally to a small electric motor, and more specifically to a small electric motor wherein a lubricant is coated on the contact surface of a commutator fixedly fitted to a rotor rotating shaft.
2. Description of the Prior Art
In a small electric motor having metallic brushes for making contact with a commutator to feed drive current to a rotor, it is a common practice to apply a lubricant in the contact surface of the commutator to prevent mechanical and electrical wear due to unwanted vibration of the brushes.
In such an arrangement, a sharp edge is provided along the length of each of the brushes by bending the brush longitudinally into a V-shape in crosssection so that the sharp edge plows through the lubricant which is made of an electrically insulating material, causing the lubricant to form a thin film during the slide contact of the brush with the commutator surface.
This arrangement will be described in detail in the following, referring to the accompanying drawings.
In FIGS. 1 through 3 illustrating the conventional construction of this type of motor, reference numeral 1 refers to a large case; 2 to a permanent magnet; 3 to a rotor; 4 to a winding; 5 to a commutator; 6-1 and 6-2 to metallic brushes having slide contact portions 6-1A and 6-2A which form sharp edges by bending the brushes longitudinally into a V-shape in crossection; brush arms 6-1B and 6-2B and terminals 6-1C and 6-2C; 7 to a rotating shaft; 8-1 and 8-2 to bearings; 9 to a small case; 10 to a commutator segment more than one of which are disposed on the surface of the commutator 5 at equal spacings in the angular positional direction; and 11 to a lubricant, for example, grease, respectively.
In the large case 1, housed are the permanent magnet 2 and the rotor 3 on which the rotor winding 4 is wound. The commutator 5 is fixedly fitted to the rotating shaft 7 of the rotor 3. The metallic brushes 6-1 and 6-2 are disposed facing each other on the commutator 5, and make sliding contact with the commutator 5 at the sharp edges of the slide contact portions 6-1A and 6-2A thereof. Both ends of the rotating shaft 7 are rotatably supported by the large case 1 and the small case 9 which covers the open end of the large case 1, via the bearings 8-1 and 8-2. The lubricant 11 is applied on the portion of the commutator 5 at which the brushes 6-1 and 6-2 make sliding contact with the commutator 5.
Consequently, as the motor rotates, the sharp edges of the slide contact portions 6-1A and 6-2A plow through the lubricant 11 deposited on the commutator 5, making electrical contact with the commutator segment 10 through a thin film of the lubricant 11. In such a case, even the lubricant 11 of a non-conductive type, when formed into a thin film, becomes conductive, causing no problems in the electrical conductivity between the commutator segment 10 and the brushes 6-1 and 6-2. Furthermore, the lubricant 11 plowed away by the brushes 6-1 and 6-2 forms ridges on both sides of the sliding path of the brushes 6-1 and 6-2, exhibiting a good damping effect in preventing the brushes 6-1 and 6-2 from unwantedly jumping up from the commutator surface.
With this arrangement, no problem need arise so long as the opposing brushes 6-1 and 6-2 face each other at the same position in the axial direction. However, if the brushes 6-1 and 6-2 shift their relative position in the axial direction, as shown by dotted lines in FIG. 3, the grooves of the lubricant 11 produced by the brushes 6-1 and 6-2 are staggered in the axial direction, as shown by solid and dotted lines in the figure. This causes the brush 6-2 to form a groove by plowing through the ridge produced by the brush 6-1, or vice versa. As a result, the contact resistances of the brushes 6-1 and 6-2 with the commutator 5 vary every rotation of the motor, resulting in fluctuations in motor rotation.
Furthermore, this arrangement requires a sufficient brush pressure to cause the sharp edges of the brush slide contact portions 6-1A and 6-2A to plow through a layer of the lubricant 11.